Graphite heating element assembly and furnaces containing same



Aug. 27, 1968 E. c. THOMAS 3,399,266

GRAPHITE HEATING ELEMENT ASSEMBLY AND FURNACES CONTAIVNING SAME 3 Sheets-Sheet 2 Filed April 28, 1966 BNQ NI L 11T, il

Aug. 27, 1968 E. c. THOMAS GRAPHITE HEATING ELEMENT ASSEMBLY AND FURNACES CONTAINING SAME Filed April 28, 1966 3 Sheets-Sheet 5 United States Patent O Filed Apr. 2s, 1966, ser. No. 545,971". i y

16 Claims. (Cl. 13e-20)- ABSTRACT or 'r'rnzffrn'sCALOSIJREl An electrical resistanceiheating element assembly comprised of two coaxially positioned headers'and two cylind'rical tubular heating 'elements is described. The headers and elements are preferably made from graphite. The assembly is particularlyfuseful in the field of'high ternperature lcalcination or heat processing. The invention covers the assembly and also heating chambers employing one or more ofthe assemblies.

Background of the invention 1. Field ni'ihninveniion This invention relates to the nem ornelds of high iemn perature calcination, reaction, or heat processing wherein a product is formedfas a result of chemical decomposition, Ireactivity or recrystallization, at temperatures typic'allywell in excess .of 1000 C., as may be required, 1ncluding graphitization as implied `and'deiinedin the art andpractice of carbon technology. More specifically, this invention defines a particular. heatingV element assembly and furnaceor heating chamber for environmentally attaining the above indicated temperatures in processesrequiring continuous and voluminous transit of iin-process material. A heatingelement :assembly of singular design, herein described, makes possiblel the degrees -of'control andfacility of processing as indicated above:

H v'Slimuzry' of the imeritionI .y

f `Itis an object of this invention .to provide primarily/.a radiation, or-if so desired, a thermally conductive heat source,`"operable continuously, at temperatures as low as 200 C. or higher, butmost advantageonslyfbetween 2000 `and 2900 ,C. if so required, yet permitting the .incorporation and application of adequate amountsof loose thermal (carbon black or other) insulation, where aplurality of heating element assemblies are required in close proximity with each other.

It is a further object of this invention to provide an inductively neutral, self contained, coaxially-disposed heating element'assembly,-powered by an alternating :currentelectricalpower supply,- said heating element lassembly being of uniquedesign which, in turn, permitsfa particularly favorable highy temperature heating ory reaction chamber design as previously indicated.

It is still a further object of this invention to facilitate the provision and lconstruction of a sealed Yreaction chamber suitable for the containment l.ofneutraliring gases under super-atmospheric pressure, for example, from 0.1 to 10 p.s.i.g. It follows that such a chamber presents a high impedance to the intrusion or diffusion of atmospheric air. The heating chamber or furnace described herein is particularly adaptable to being used to-graph itize carbon vfibers or cloth but maynd vgeneral utility in several heating or heat-treating processesl and isnot yrestricted to use in the cloth' liber or clothgraphitizing field. For purposes of the present invention theheating chamber can be defined as preferably including: (a) a base; (vb) an internal shell consisting `of walls and a top fabricated from graphite; (c) internal loose `thermal insulating Vmaterial contained within said shell bounded 3,399,266 Patented Aug. 27, 1968 ICC by the walls thereof and the base; (d) external loose thermalv insulating material surrounding the walls and on top of said shell; and (e) external thermally insulating structural walls surrounding said external loose thermal insulating material. The internal shell ofthe heating chamber will also typically possess at least one `chimney opening therein (extending through the loose thermal insulation on the top of said-shell), particularly if the heating chamber is used to process-materials which evolve vapors and/or gases upon being heated. The heating chamber is also heated in a very specific manner by means of at least one electricalresistance heating element assembly coupled into said heating chamberithrough one of the external walls, through the external loose thermal insulating material and through a Wall ofthe internal shell in order to provide heat Within said "shell, saidf assembly comprising two independent, coaxially positioned carbonaceous headers, preferably graphite, an outer tubular primary header and an inner tubular secondary header each of which at an end thereof outside the external walls is connected to an external power source, said headers being electrically insulated from each other, each of said headers also being connected to a high temperature, electrically conductive, cylindrical carbonaceous tubular heating element, which elements extend into the internal shell, said elements being substantially coextensive and coaxially disposed, and spaced from each other and electrically coupled to each other at their ends, remote from the headers, by an electrically conductive carbonaceous spacing plug which closes the annulus between the aforementioned heating elements. Preferably the heating chamber will be heated by at least two of the electrical resistance heating element assemblies as just described, and detailed arrangements as to how this typically is accomplished are illustrated in the drawings.

Brief description of the drawings In order to facilitate the description of both the heating chambers encompassed herein as well as the structure and operation of the heating element assembly, its application in a specilic heating process and apparatus is described herein with -particular reference to the drawings. In particular, the process in reference is the thermal conversion of a carbon fiber product to the graphitic state. A specific arrangement of the equipment used in this process, including the heating element assemblies, is shown in FIGS. 1 and 2 of the drawings, which are vertical and horizontal sectional (and partly schematic) views, respectively, of the cloth graphitizing apparatus. Details 0f the heating element assembly are shown in FIGS. 3-5 wherein FIG. 3 is a side-sectional view (broken) of the heating element assembly, FIG. 4 is a side view (broken) of the heating element assembly as well as of the electrical connections made to same, and FIG. 5 is an end view of the heating element assembly and electrical connections of FIG. 4 taken along the lines S-S of FIG. 4.

Detailed description of the drawings and' of the nlvention The cloth graphitizing heating apparatus or assembly illustrated in FIG. l comprises gas tight containers or boxes 1'5 and 15a for containing the carbon cloth to be processed and the graphite cloth product; a heating or reaction chamber or hot zone 16 which contains the heating elements; and passageways 17 and 17a leading into and out of the hot zone. As illustrated in FIG. 2, the heating elements are connected to an external power source by suitable connections such as copper bus bars 12 and 12a. Structural members or plates and bricks, etc., and insulating materials, as previously indicated and as illustrated in the drawings, are used in the fabrication of the heating chamber `or furnace. More details of the 3 entire` cloth heatingassembly or cloth graphi'tizing furnace are given in separatej'patent application Ser. No. 546,098 led of even date hereofwherein the heating assembly for graphitizing cloth is specifically claimed, and described in all of its details. FIGS. 1 and 2, however, Iand the foregoing discussion of same, should be sufficient to teach andy explain, to those skilled in the art, the utility of the heating elements of the present invention, and also provide some guidelines for the use of such heating elements in heating chambers generally, viz. as well as in a cloth graphitizing furnace assembly or apparatus as illustrated in FIGS. l and 2 but not restricted thereto.

1 vWith particular reference now being made to FIG. 3, t

the high temperature heating element assembly comprises two electrically conductive header components, machined froml graphite, a primary header 1,v and a secondary header. 2, coaxially positioned, but electrically insulated from each other by layers of a compliant electrical insulator 10 such as Refrasil (a registered Trademark of Iy-I.' I. Thompson Company for a woven fabric made from ysilica fibers). The high temperature heating element assembly also comprises two lengths of high temperature lelectrically conductive carbonaceous tubing, i.e. graphite, coaxially disposed, with the primary element (viz. the element connected to the primary header 1) 3 surrounding the secondary element 4, symmetrically spaced, one about the other by an electrically conductive carbonaceous, i.e. graphite, plug 6. The aforementioned, therefore, describes an electrically coaxial circuit with the primary and secondary tubular heating elements 3 and 4 connected in series. The primary and secondary headers 1 and 2 serve to complete the series circuit, to provide a means to support the heating elements in proper relation, one-to-the-other, and to provide a means to mount the heating element assembly in and through the walls of the heating chamber. Tubular elements 3 and 4 are typically from about 2 to about 5 times as long as the headers into which they are connected.

Another important feature of the heating element assembly is an axial bore 5, which extends from the terminal end of the secondary header 2 through the secondary header and registers with a bore 5a in the secondary element 4. These registering axial bores define a substantially central longitudinal cylindrical channel which provides transit for a neutralizing gas such as argon, helium, nitrogen, etc., through gas inlet 20 in secondary header 2. The bore terminates in an electrically conductive carbonaceous, i.e. graphite plug 7, and the gas vents through a hole or opening 8 (which typically is near said plug) on into the annulus 24 between the primary and secondary heating elements. Further passage -for the gas out of the annulus is provided through an opening or hole 9 (in the primary header tubular heating element, typically near the primary header) venting the gas through the primary element into the chamber being heated. The gas train so defined permits a positive pressure of a neutralizing gas within the corporate body of the element, thereby excluding air which might seep into the assembly through the seal 11. (This seal may be made with Refrasil cloth cemented with an electrically insulative material such as an epoxy resin.) Instead of electrically conductive plugs 6 and 7 being separate members it should be appreciated that they may be machined and suitably shaped, for example from graphite, as a single member. Legends 12 and 12a show typical connections of copper bus to the primary and secondary headers 1 and 2. Primary and secondary headers 1 and 2 typically have cylindrical shouldered recesses 18 and 19, respectively, therein into which the primary element 3 and the secondary element 4 may be press-fitted. These members may, of course, be connected in other suitable ways, such as by threaded connections. Holes may be provided in the headers t-o facilitate their connection to the bus bars such as by means of bolts 30. This arrangement may be used with AC or DC current. When used with alternating current, the aforementioned benefits. derived from'such an electrically noninductive system are, namely, that where substantial amountsv of power are expended, thel power yfactorof the circuit can be maintained near unity, Vthereby minimizing electrical power losses from what is' generally referred to as the wattless component in. an inductive type circuit.

Although the foregoing paragraphdescribes a highly desirable condition of operation that would be inadvertently derived from the noninductive loop heating element, it is by far the least important insofar as the particular heating chamber and heating element assembly under consideration are'concerned. The important considerations follow, as a specific application of such an assembly to a particular process is described.

As an example, two of the A'aforementioned heating element assemblies were employed as illustrated in the furnace arrangements ofFIGS. 1 and 2 in a continuous process wherein carbon liber products were converted to graphite liber products by thermal decomposition and purification at graphitizing temperatures. The outer tubular heating elements (those connectedA to the outer tubular primary headers) were in direct contact with, on top of, and supported by the loose thermal insulating material contained within the internal shell. The headers 1 and 2 were water cooled. It will be noted that in the illustrated arrangement the two heating element assemblies entered the heating chamber through opposite walls, but that with regard to each single assembly, its primary and secondary headers were both on the same side of the furnace. No processing or apparatus difficulties attributable t-o. the heating element assemblies or heating chamber arrangements were encountered. When efforts were made to use other types of resistance heating elements, the electromagnetic iields induced' by the heavy currents required therein, in order to attain graphitizing temperatures eventually caused serious overheating in the insulating, materials used within the internal shell and surrounding same (inattempting to contain temperatures through 2900 C.). At these temperatures (and even lesser) the bound electrons in .any available or well known ceramic`.refractory insulating materialsbecorne free and mobile, and materials which are normally insulators become electrically conducting, thereby responding to the aforementioned alternating electromagnetic fields. In the elements` under discussion, however, the electromagnetic fields were neutralized or cancelledby the noninductive loop principle previously described. Also, as` a result of this, in another furnace arrangement anumber kof such element assemblies were used inclose proximity to one another, and at no time were there either conductivemagnetic fields, or differences in potential which motivated the thermally freed electrons in the superheated ceramic insulating materials.

In short, the *heating element assemblies of this invention permittedhthe use of various common insulating materials, through and including carbon blacks, which are excellent thermal barriers both from the standpoint of thermal conduction, as well as high temperature `radiation. f n a It should further be pointed out and emphasized that eachheating element assembly'so described .is a heat source entity in itself, permitting close temperature control through readily adjustable aIternatingcurrent power suppl-ies, with. such current levels as may :be required.

A further benefit derived from this heating'element assembly design is the condition illustrated in FIG. 2 wherein one end of the assembly terminates inside the heated oavity or reacting chamber of the particular furnace within which it is used, as shown in FIG. 2. (Typically -in this arrangement the tubular heating elements ex tend through one'wall of the internal shell for a substantial distance toward theopposite wall of the shell, but short of same.) Since this element 'assembly has' a normaLthermalIy induced thermal expansion, it can be permitted free expansion at its terminal end without danger or concern of its pushing out of a retaining wall at the opposite end (or side) of the furnace;

'This'latter feature becomes very important where the refractory chamber `'must be kept reasonably well sealed to atmospheric air'. This was particularly true in the case of the aforementioned fiber product-s furnace illustrated in FIGS. 1 and 2 and previously described. yIt was thus possible to impress a slightly superatmospheric pres-sure in the reaction chamber or hot zone of the furnace and inhibit the diffusion, or 'the chimney effect conduction of air into the hot lzone or vital cavity of the fiber products processing lfurnace. Although for very high temperature applications heating chamber designs previously described will be employed, it is possible to change these designs in several details if lower temperature operations (for example, between l350 and. 1000 C.) are desired. In such cases it is possible to use carbonaceous materials other th-an graphite for construction of certain portions or all of the heating element assembly. (By carbonaceous as used here and in other portions of the specification and claims is meant a fabricated material, typically formed, for example, by extrusion or `m'oldin lf-rom a mixture of carbonaceous particles (e.g. graphite particlesor calcined petroleum coke particles) and la binder such as pitch. The bodies may also be made and 'formed from other starting -materials such as raw petroleum coke and a bin-der or raw petroleum coke plus a plasticizer, etc. The formed |articles are then typically packed and baked (eg. heated to` about 700,-1000"` C.) in a baking oven. Preferably these formed-and baked articles are then graphitized by heat-treatment to temperature between about 2000 and 3`0O C. In general, Vthe Ima'chineaibility of the article will be greater the higher the final heat-treatment temperatu'rel) However, as aforesaid, formed carbonaceous materials other than graphite xarticles may he used as cornponents of the `heating element assembly. For example, it is possib-le to construct the concentric tubular element portions of the assembly of a semi-graphite which would :have a Vhigher resistance than graphite. Therefore, for any given power output the voltage required would be greater for the semi-graphite element thereby providing a more precise or lhigher degree of control. For purposes of this invention, therefore, the term "carbonaceous is meant to connote articles such as just described regardless of whether they are baked, semi-graphitic or graphite articles. i y

It is also possible `to omit the use of rnuch of the loose insulating material and the lheating chamber can be insulated simply by means ofthermally insulating refractory walls, rather than radouble set of wal-ls 'with loose thermal insulation lin between. The use `of the heating element assembly with such an. altered heating chamber design is also very advantageous, particularlyV in connection with hermetically sealing the walls of the furnace where the assembly goes through, and there-fore such alternative designs in combination with one or more ofthe heating element assemblies described herein, are also considered apart of this invention. Such an arrangement would be useful, for example, in a tilted tunnel kiln for the continuouscalcination of petroleum coke.

It should also `be appreciated that the heating chamber embodiment illustrate-d in FIGS. 1 and 2 may be varied in. some details depending upon-the particular heating process,-.in..mind.For example, passageways 17 and 17a may be varied'in height and/'or widthtoaccomrncdate the heat-treating or `processingof thicker materials. Passage'Ways-17 and` 17a could alsoibe modified so las to bridge across the hot zone 16 as a support means for transferring the material 4being processed.

Although I have described my invention w-ith a certain degree of particularity, it is understood that the present disclosure has been made only by way of example an-d that numerous changes in the details of construction and the combination and arrangement of parts may be resorted to without departing from the spirit and the scope of the invention as hereinafter claimed.

I claim:

1. An electrical resistance heating element assembly comprising two coaxially positioned carbonaceous headers, an outer tubular primary header and an inner tubular secondary header each of which at one end thereof is adapted for connection to an external power source, said headers being electrically insulated from each other, each of said headers also being connected to a high temperature, electrically conductive, cylindrical carbonaceous tubular heating element, said elements being .substantially coextensive and coaxially disposed, and spaced from each other, and electrically coupled to each other at their ends, remote from the headers, by an electrically conductive carbonaceous spacing plug which closes one end of the annulus between the lheating elements.

2. An electrical resistance heating element assembly according to claim 1 wherein said headers possess cylindrical shouldered recesses therein into which said tubular heating elements are press-fitted.

3. An electrical resistance heating element assembly according to claim -1 wherein said tubular heating elements are from about 2 to about 5 times as long as the headers to which they are connected.

4. An electrical resistance heating element assembly according to claim 1 wherein said secondary header and said cylindrical tubular heating element connected to said header contain a substantially central longitudinal cylindrical channel which extends from the terminal end of the secondary header to a carbonaceous electrical conductor which. closes the end of the channel in the secondary header tubular heating element.

5. An electrical resistance heating element assembly according to claim 4 wherein said electrical conductor which closes the end of the channel in the secondary header tubular heating element and Said spacing plug which closes one end of the annulus between the heating elements are fabricated from a single carbonaceous member.

6. An electrical resistance heating element assembly according to claim 4 wherein said headers are spaced from each other by substantially gas-tight electrical insulation, wherein said secondary header contains a gas inlet which communicates with said central longitudinal channel, wherein said secondary header tubular heating element has an opening therethrough near its end which is closed with the carbonaceous electrical conductor, and wherein the primary header tubular heating element has an opening therethrough near the primary header.

7. In combination, (A) a heating chamber including: (a) a base; (1b) an internal shell consisting of walls and a top fabricated from graphite; (c) internal loose thermal insulating material contained within said shell bounded -by the walls thereof and the base; (d) external loose thermal insulating material surrounding the walls and on the top of said shell; and (e) external thermally insulating structural walls surrounding said external loose thermal insulating material; and (B) at least one electrical resistance heating element assembly coupled into said heating chamber through one of the external walls, through the external loose thermal insulating material and through a wall of the internal shell in order to provide heat within said shell, said assembly comprising two coaxially positioned carbonaceous headers, an Outer tubular primary header and an inner tubular secondary header each of which at an end thereof outside the external walls is connected to an external power source, said 7 headers being electrically insulated from each other, each of said headers also being connected to a-high temperature, electrically conductive, cylindrical tubular lcarbonaceous heating element whichelements extend into the internal shell, said elements being substantially coextensive an-d coaxially'disposed, and -spaced from each other and electrically coupled to each other at 'their ends remote from the headers by an electrically conductive carbonaceous spacing plug which closes the annulus between the heatin-g elements.

8. A combination according to claim 7 Whereinthe tubular carbonaceous heating elements extend through one wall of the internal shell, substantially near, =but short of the opposite wall. l

9; A combination according to claim 7 wherein the tubular carbonaceous heating element,'which is connected to the outer tubular primary header of the electrical resista ance heating element assembly, is in direct contact with, on top of and supported =by the loose thermalnsulating material contained within said internal shell."

10. A combination according to claim 7wherein the ends of the primary and secondary headers, which are outside the external wall and connected to the external power source, are water cooled.

11. A combination according to claim 7 wherein the tubular elements connected to the primary and secondary headers are from about 2 to a'bout 5 times as long as the headers to which they are connected.

12. A combination according to claim 7 wherein said secondary header and said cylindrical carbonaceous tubular heating element connected to said header contain a substantially central longitudinal cylindrical channel which extends from the terminal end of the secondary header to an electrical conductor which closes the end of the channel in the secondary header tubular heating element.

13. A combination according to claim 12 wherein said headers are spaced from each other by substantially gastight electrical insulation, wherein said secondary header contains a gas inlet which communicates with said central longitudinal channel, wherein said secondary header tubular heating element has an opening therethrough near its end which is closed with the electrical conductor, and wherein the primary header tubular heating element has an opening therethrough near the primary header.

' v14..-'A combination-according to c1aim-'7'wherein..the graphite" top of said internal shell possesses at -least one vapor stack opening therein, which extendsthrough the loose thermal insulation on thetop-ofsaid'she1l,-provid ing an avenue `of escapeforany undesirable ash vand' 'hy' drocarbon vaporswhich are normal products ofdecomposition of the material=y processed in theheating chamber.

-15. A combination according to claim7 wherein' said heating chamber has more than o'ne heatng element fas-` sembly coupled into same' and wherein fheaders vof '-said assemblies enter the heating'charnlberl through opposite Walls.: .A '1 ";.l

16; In combination, f(a) a heating chamber comprising a' shell consisting of thermally insulating structural-Walls and top; and (b) at least-one' electrical resistancewheating element assembly coupled Iinto ysaidheatin'g chamber through one of the walls of the shell in orderIto-tprfovide heat within said'shell,l said assembly comprising'two-'c axially positioned carbonaceoushcade'rs,"an outer'tubuj lar primary header andan inner' tubular secondary header each of' which at an en d thereofvutside thewall isco'nnected to an external power source, said headers being electrically insulated `from each lother, eacl1l of said head ers also `being connected toa high temperature,'electrical ly conductive, cylindrical carbonaceous tubular heating element, which elements extend intotheinternal shell, said elements being substantially coextensive andlcoaxially disposed, and spaced from each other,and electrically coupled to each other at their ends, .remote from the/headers,l byt an electrically conductive carbonaceous spacing plug Jwhich closes one end of the annulusA between the heating elements. f

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